lazily applied addCustomCSS not accepted

[MarcusNotheis]

Describe the bug
We have a scenario where we load an app extension (components) from another URL. The components from this URL are containing e.g. a ui5-select and a addCustomCSS call. Although the addCustomCSS call is executed and the style is registered in the customCSSFor map in CustomStyle.js, the styles are not applied.
I also tried to force a rerender using _invalidate() or _updateShadowRoot() on the ui5-select but it had no effect.

To reproduce
Steps to reproduce the behavior:

1. Open the codesandbox
2. Press apply styles and rerender
3. Nothing happens (I would have expected that the select background is now red.
4. Optional - if you move the addCustomCSS outside of the callback everything works as expected.

Isolated example
https://codesandbox.io/s/epic-heyrovsky-q4m0j?file=/src/index.js

Expected behavior
I would expect that custom styles are applied when components are rerendered. (Updating the affected components can be in the responsibility of the user IMO, but would be nice if the framework could do that as well).

Context

• UI5 Web Components version: 1.0.0-rc.8

[vladitasev]

Hi @MarcusNotheis ,

We have identified the issue and are working on a fix. However, we're considering this a feature - making custom CSS dynamic.

Regards,
Vladi